NetNotes
the sharpness of the diffraction spots (all other things being equal). (2) Keep the exposure times reasonably short (<0.1 s) in the first instance. Check the maximum intensity against the dynamic range of the camera (your 14-bit camera may have a realistic maximum intensity of 14000 counts per pixel). Find the exposure time that will give get about 10000 counts maximum and then use multiple exposures to boost the total acquisition time. In Digital Micrograph this will be via “Camera>Acquisition Parameters>Acquire” scroll down menus. On the “GIF CCD Setup” tab you will see a panel labeled “Processing” and “Frame Sum(# frames)”. Tis is usually set to “1” for a single shot image. Set this to 10 or more for a high signal diffraction pattern. I’ve used this method a number of times to get reasonably sharp diffraction patterns with good success. Jon Barnard
jsb43@hermes.cam.ac.uk Fri Apr 1
TEM:
viewing CdHgTe2 Has anyone had experience looking at CdHgTe2 on a TEM? I expect
that using a cryo stage would be the best way to inhibit contamination of the scope from the volatile components. Is this the case, and are there other methods that would allow this compound to be imaged without damage to the scope? Bill Tivol
william.f.tivol@aero.org Fri Mar 25 I’m not familiar with the compound, but have heard wonderful
(terrible?) stories about what can happen to a high voltage power supply when Hg becomes Hg vapor and rather conductive. I would think that if the Hg is tightly bound chemically, then it shouldn’t be an issue, but if you have any doubts, I think cryo might be a good bet, or at least a hedge. Te biggest problem seems to be its migration aſter being vaporized. If you don’t get rid of all of it, you’ll blow up your HVPS again. Ken Converse
kenconverse@qualityimages.biz Fri Mar 25 Before we have too much panic, it might help to remember that
for many years, JEOL used a mercury diffusion pump in their tandem pumping system for their best 100 and 200 keV TEMs. Such devices intentionally produce massive amounts of mercury vapor. Jim Pawley
jbpawley@wisc.edu Fri Mar 25 Massive amount of Hg vapor in the room?
nizets2@yahoo.com
Mon Mar 28 Your big problem will be making specimens that do not have
point defects galore all over the surface. I believe Tony Cullis did a lot of CdHgTe2 work back in the 80’s, and that was one of the motivators to develop iodine milling. I don’t recall any problems with the compound volatilizing. John Mardinly
john.mardinly@
asu.edu Fri Mar 25
TEM:
magnetic particles I have a request for imaging magnetic nanoparticles in TEM. Te
particles are currently in dry powder form. Tey are easily attracted to a small permanent magnet. My initial reaction is to refuse the request for fear of coating the objective lens with magnetic particles. However, perhaps someone has successfully imaged such and can advise a safe method for introducing them into the TEM. Roger A. Ristau raristau@
ims.uconn.edu Tue Apr 26 No problem with magnetic nanoparticles—this is one of my
specialties, and I do it all the time with no problem at all. Just take care that they are dispersed, by creating a suspension in water, shaking all particles apart by a supersonic bath and then drip a droplet on a holey carbon grid, before they re-coagulate. Te smaller the individual particle, the less the magnetic force acting on it. Te
68
force is proportional to the integral over the particle’s volume taken over its magnetization times the gradient of the magnetic field. (You may want to check
http://en.wikipedia.org/wiki/Magnetic_force_ microscope). Te gradient is usually small, although the total field is around 1–2 Tesla for a 300 kV microscope, as is the volume of the specimen. Tus the total force is small. Care should be taken for extremely high-magnetic moment particles above a certain size. I also recommend to switch off the lenses (objective is most important) during specimen insertion, as the stray fields at the vacuum transfer port may be rather large. Joe Zweck
Josef.Zweck@physik.
uni-regensburg.de Tue Apr 26 Should you find a way of mounting the particles in safety may I
help on the observation side? With any magnetic material it is useful to reduce the level of the magnetic field in the objective lens. Tis is easily done using the eucentric stage to raise the specimen within the pole piece. With a “normal” specimen adjust the eucentric system so that as you change the Z prime you need to turn the objective lens anti clockwise. Tere are a few millimeters available to you and this will have quite an effect upon the lens field. Remember your magnifications will be lower and your contrast will be higher. Te magnification change due to the weaker lens and the contrast due to increasing the specimen to aperture distance and increasing aberrations. Whilst I rarely carry out this action in this direction but oſten obtain more than the manufacturer claims in resolution by working the Z prime to lower the specimen within the lens, increasing the lens field and reducing its aberrations. Steve Chapman
protrain@emcourses.com Tue Apr 26 When I was at Western Digital, the SEMs would periodically
need to have the final pole piece cleaned of magnetic particles, and these samples were solid sputtered films with obvious no loose particles and sub 10 nm grain size, not powders. Be very careful with loose particles. A. John Mardinly
john.mardinly@
asu.edu Tue Apr 26 In order to avoid any problem with deposition of nanoparticles
on OL pole piece you can adopt the extreme solution of embedding the particles in a resin and aſterwards processing the sample for TEM as if it were a solid sample. It is time consuming, but is the safest. I use that procedure when I do TEM on small grains of magnetic alloys. It takes time and effort to prepare the sample, but it guarantees no deposition of particles on OL pole piece will occur. Corneliu Sarbu
crnl_srbu@yahoo.com Sat Apr 30
TEM: compressed air We have a major problem with compressed air for our Tecnai G20
TEM. I’ll try to be as precise as possible but in the end I think that the answer should be quite standard. I can remember that in the end of last year we had a discussion on the list about how important it was to make sure that the compressed air arriving at the TEM is dry to avoid wasting the valves. I took this message seriously and reviewed our own system but I found it to be well conceived since we had (1) a large pressure container which can collect a large volume of water between the compressor and the pipes. (2) a kind of filter aſter the container, although I admit I don’t really know its function. I guessed it was there to retain water. Te problem itself: about 2 weeks ago during a routine check we noticed that the small container located in the flow meter rack, which purpose is to collect the rest of condensed water from the pipes and which remained completely dry for 5 years was completely full with water. At that time it seemed that we came in time and the water did not overflow in the tubing to the microscope parts. Te container
www.microscopy-today.com • 2011 July
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84